The Secure Digital (SD) standard was introduced in August 1999 as an improvement over Multi-Media Cards (MMC). The Secure Digital standard is maintained by the SD Association (SDA). SD technologies have been implemented in hundreds of brands, across dozens of product categories, and in thousands of electronic devices. SD is often used in nonvolatile memory cards. These memory cards are extensively used in portable devices, such as mobile phones, digital cameras, GPS navigation devices, handheld consoles, and tablet computers. The Secure Digital format includes four card families available in three different form factors. The four families are the original Standard-Capacity (SDSC), the High-Capacity (SDHC), the eXtended-Capacity (SDXC), and the SDIO, which combines input/output functions with data storage. With the exception of SDIO, these standards can be implemented in three form factors: original size, mini size, and micro size. Electrically passive adapters allow a smaller card to fit and function in a device built for a larger card.
The Secure Digital Input Output interface standard is a more recent extension of the SD specification to cover I/O functions. The SDIO standardized interface has a low pin count. SDIO cards are only fully functional in host devices designed to support their input/output functions (typically Personal Digital Assistants (PDAs), but occasionally laptops or mobile phones). These devices can use the SD slot to support Global Positioning System (GPS) receivers, modems, barcode readers, FM radio tuners, TV tuners, Radio Frequency Identification (RFID) readers, digital cameras, and interfaces to Wi-Fi, Bluetooth, Ethernet, and IrDA. SDIO cards support most of the memory commands of SD cards. SDIO cards can be structured as eight logical cards, although currently, the typical way that an SDIO card uses this capability is to structure itself as one I/O card and one memory card.
SDIO and SD interfaces are mechanically and electrically similar. Host devices built for SDIO cards generally accept SD memory cards without I/O functions. However, the reverse is not true; host devices need suitable drivers and applications to support the card's I/O functions. Inserting an SDIO card into any SD slot causes no physical damage nor disruption to the host device, but users may be frustrated that the SDIO card does not function fully when inserted into a seemingly compatible slot. What is needed is a better way to interact with SDIO devices.